The impact on relative risk estimates of inconsistencies between ICD-9 and ICD-10

• Published in: Occupational and environmental medicine (London, England) Vol. 63; no. 11; pp. 734 - 740
• Main Author: Richardson, D B
• Format: Journal Article
• Language: English
• Published: London BMJ Publishing Group Ltd 01-11-2006; BMJ Publishing Group; BMJ; BMJ Publishing Group LTD; BMJ Group
• Subjects: Biological and medical sciences; Cancer; Cause of Death; Causes of death; Classification; Coefficients; Cohort Studies; Death; Death Certificates; epidemiologic methods; Epidemiology; Estimates; Estimation bias; Fatalities; General aspects; Health risk assessment; Humans; Information classification; International Classification of Diseases - standards; International Statistical Classification of Diseases; Medical sciences; Methodology; Mortality; Occupational Diseases - mortality; occupational mortality; Original; Public health. Hygiene; Public health. Hygiene-occupational medicine; Pulmonary heart disease; Revisions; Risk; Sensitivity and Specificity; United States > US; Human; Disease; Methodology; Estimation; Mortality; Revision; International Classification of Diseases 10; Death certificate; Classification; Risk factor; Death; Methodological bias; List; Causality; Comparative study; Public health
• ISSN: 1351-0711; 1470-7926
• DOI: 10.1136/oem.2006.027243

Background: The 10th revision of the International Classification of Diseases (ICD) represents a major change in the ICD system. This paper investigates the impact on relative risk estimates of inconsistencies in outcome classification between ICD-9 and ICD-10, including scenarios in which occupational exposure levels are correlated with year of death (and therefore with the ICD revision in effect at death). The setting of interest is a cohort mortality study in which follow up spans the periods during which ICD-9 and ICD-10 were in effect. The relative risk estimate obtained when death certificates are coded to the ICD revision in effect at time of death is compared to the relative risk estimate that would be obtained if all death certificates were coded to a consistent ICD revision (that is, ICD-10). The ratio of these relative risks is referred to as the coefficient of bias. Methods: Simple equations relate the coefficient of bias to the sensitivity and specificity of the classification of decedents into categories of cause of death via ICD-9 (treating classifications based upon ICD-10 as the standard). Bridge coded mortality data for 2 296 922 decedents (that is, death certificates coded to ICD-9 and ICD-10) are used to derive estimates of sensitivity and specificity by category of cause of death. Numerical examples illustrate the application of these equations. Results: Estimates of the sensitivity of classification of decedents into categories of death defined by ICD-9 ranged from 0.26–1.00. Specificity was above 0.98 for all categories of cause of death. Numerical examples illustrate that inconsistencies in outcome classification between ICD-9 and ICD-10 may have substantial impact on relative risk estimates if there is a strong relation between exposure status and the proportion of deaths coded to a given ICD revision. Conclusions: For analyses of mortality outcomes that exhibit poor comparability between ICD-9 and -10, it may be prudent to recode cause of death information to a standard ICD revision in order to avoid bias that can occur when exposures are correlated with the proportion of deaths coded to a given ICD revision.